Translesion synthesis (TLS) DNA polymerases (Pols) help maintain the continuity of the replication fork by mediating the bypass of DNA lesions. To elucidate the mechanisms that govern replicative bypass of DNA adducts in eukaryotes including in humans, the proposed studies will utilize the yeast Saccharomyces cerevisiae. Since DNA repair and lesion bypass processes have been highly conserved between yeast and humans, the knowledge gained from these studies will be directly applicable to the understanding of TLS mechanisms in human cells. In Aim 1, the role of DNA damage-induced checkpoint kinase-mediated Rev1 phosphorylation in Pol6-dependent TLS will be studied using a combined genetic and biochemical approach. In particular, the hypothesis that Rev1 phosphorylation enhances the efficiency of TLS by Pol6 will be tested. In Aim 2, studies will be done with the novel 4-subunit Pol6, comprised of Rev3, Rev7, Pol31, and Pol32 subunits, that we have purified. Specifically, we will examine its lesion bypass properties and will determine whether it physically and functionally interacts with PCNA. In Aim 3, the role of Slx4, which we have recently identified as a component of Pol6-dependent TLS and which associates with Rev1 to form the Rev1-Slx4 complex, will be defined. Genetic and biochemical studies will be conducted to determine the manner in which the Rev1-Slx4 complex modulates Pol6 function. In Aim 4, studies will carried out to test the hypothesis that the Rad6-Rad18-Rad5 complex functions as a structural element in TLS to promote the assembly of TLS Pols and their associated protein factors at DNA lesions. In Aim 5, TLS through site-specific DNA lesions carried on a plasmid will be examined so as to determine the contributions of different protein complexes to the TLS reaction. In particular, we will test the hypothesis that in addition to its structural role in the assembly of TLS Pols, the Rad6-Rad18-Rad5 complex functions as a mediator in TLS wherein it coordinates the actions of the TLS Pols with that of the replicative Pol. The roles of PCNA ubiquitination and of the PCNA binding and ubiquitin binding domains present in TLS Pols will also be analyzed in these studies. By helping ensure the continuity of the replication fork and by promoting error-free replication through a large variety of DNA adducts that result from the attack of free radicals and reactive oxygen species generated during normal cellular oxidative reactions and from exposure to chemical and environmental carcinogens, the TLS processes provide an important means for minimizing the mutagenic effects of DNA damage and for the suppression of carcinogenesis in humans. For instance, the inactivation of Pol7 in humans causes highly elevated levels of skin cancers. The proposed studies are highly relevant for cancer etiology, as they will be important for elucidating the mechanisms of TLS in human cells and for revealing the contributions that TLS processes make to genomic integrity and to the prevention of the mutagenic and carcinogenic consequences of DNA lesions.